The present invention relates to the field of motor-vehicle safety and concerns more particularly an electropyrotechnic igniter.
Over the last 30 years, many electropyrotechnic igniters have appeared and are widely used for constructing either the ignition devices for gas generators intended for inflating airbags for protecting the occupants of a motor vehicle or the gas microgenerators included in seat-belt pretensioners.
Conventionally, electropyrotechnic igniters comprise, on one side, metal pins connected to a source of electric current and between which a ferrite core is generally placed and, on the other side, a resistive heating element which is attached to the said metal pins and which is covered with a pyrotechnic initiating composition. As described in Patent Application FR 2,704,944 or in its corresponding patent U.S. Pat. No. 5,544,585, the resistive heating element may consist of a resistive heating strip incorporated into a printed subcircuit.
An electropyrotechnic igniter of elongate shape and of small diameter, allowing the metal pins and the ferrite cores to be omitted by integrating their functions into a complete printed circuit, has also been proposed, in Patent Application FR 2,760,525. However, because of its particular geometrical configuration, this igniter cannot be used instead of the igniters commonly used in gas generators and in seat-belt pretensioners.
Moreover, the greatly increased number of xe2x80x9cairbag modulesxe2x80x9d incorporated into a motor vehiclexe2x80x94an xe2x80x9cairbag modulexe2x80x9d consisting of a particular entity containing a gas generator associated with an airbagxe2x80x94results in an increased number of igniters employed. In order to avoid having to make individual electrical connections between each igniter and the source of electric current, which would mean excessively high installation costs and too much space being taken up, it is therefore desirable to incorporate, into the gas generators of the various airbag modules, igniters that can be connected to a central control unit via a hard-wired circuit so as to limit the number of cables placed in the vehicle. But this results in several problems to be solved consisting of the fact that:
such an igniter must have an overall external size similar to that of an igniter normally used, so as to be able to replace the latter in airbag modules;
in the event of a collision, the central control unit may not be able to deliver enough electrical energy to cause the various igniters included in the airbag modules to be initiated; and
depending on the nature and the severity of the collision for example, it is desirable to be able to choose to trigger only the airbag modules allowing a suitable protection to be provided.
Those skilled in the art are therefore always seeking electropyrotechnic igniters capable of being integrated into a hard-wired circuit connected to a central control unit and having an overall external size similar to that of conventional igniters.
The subject of the invention is specifically to provide such an igniter and therefore relates to an electropyrotechnic igniter comprising a body having a resistive heating element, a pyrotechnic initiating composition and a pyrotechnic ignition composition, the said resistive heating element being electrically connected to at least two external electrodes attached to a printed-circuit substrate produced in the form of an electronic card on which conducting tracks are provided, characterized in that:
i) a transverse separating wall divides the inside of the body into a downstream chamber, containing the resistive heating element, the pyrotechnic initiating composition and the pyrotechnic ignition composition, and an upstream chamber, containing the electronic card;
ii) electrical connection means pass through the said transverse separating wall and connect the conducting tracks on the electronic card to the resistive heating element;
iii) a means of intercommunicating and of triggering coded information as well as a means of storing electrical energy are connected to the conducting tracks.
It therefore follows that:
this electropyrotechnic igniter can be used for the same purpose as a conventional igniter since the use of an electronic card on which the various electronic components are integrated makes it possible for the said igniter forming the subject of the invention to have an overall external size similar to that of a conventional igniter;
the use of such an igniter in each of the various airbag modules incorporated into the vehicle and connected via a bus-type hard-wired circuit to a central control unit makes it possible, on the one hand, to leave to the central control unit the choice of triggering only the airbag module or modules capable of providing the occupant with effective protection depending on the impact and, on the other hand, of no longer requiring the central control unit to deliver the amount of electrical energy needed to cause such an igniter to be initiated. This is achieved by the presence in each igniter, on the one hand, of a means of storing electrical energy which is periodically supplied with a low-intensity electric current emitted by the central control unit and, on the other hand, of an intercommunicating and triggering means which is capable of detecting an item of coded information coming from the central control unit and of giving the said means of storing electrical energy the command to deliver the amount of electrical energy stored until then. This allows the resistive heating element to be heated by the Joule effect so as to initiate the pyrotechnic initiating composition.
Preferably, the intercommunicating and triggering means will be a specific integrated circuit and the coded information which flows between the central control unit and the said intercommunicating and triggering means will consist of trains of specific electrical pulses. In the present application, the coded information will comprise both the information constituting the commands for triggering the various igniters and the information allowing the central control unit to be sure of the reliability of the electronic components contained in each igniter.
Also preferably, the intercommunicating and triggering means is placed on one of the two plane faces of the electronic card and the means of storing electrical energy is placed on the other plane face.
Advantageously, the transverse separating wall is produced using a metal piece with several perforations, each of these perforations having a glass side wall. Advantageously, the metal piece has two perforations and the electrical connection means consist of two metal connection pins, each of the latter being inserted into one of the two perforations in the metal piece and having a first end attached to the resistive heating element and a second end soldered to the conducting tracks. This transverse separating wall therefore makes it possible to ensure sealing between the upstream chamber and the downstream chamber, before and after operation of the igniter, but also to electrically isolate the metal connection pins from each other.
Also advantageously, the electronic card is placed at right angles to the transverse separating wall. In order to increase the mechanical strength of the electronic card and of the electronic components fastened onto it, it is desirable to cover the whole assembly with an overmoulding compound or with an encapsulation compound.
Preferably, the resistive heating element consists of a thin-film resistive bridge resting on the metal piece, the said thin-film resistive bridge being made of tantalum nitride with a thickness of between 0.01 xcexcm and 1 xcexcm. The resistive bridge may also consist, for example, of a layer of a nickel-chromium alloy. Also preferably, the pyrotechnic initiating composition is a lacquer based on lead trinitro-resorcinate which covers the resistive bridge. The said pyrotechnic initiating composition may also consist, for example, of dinitrobenzofuroxan salts.
Preferably, the external electrodes consist of external metal pins which are placed in the extension of the electronic card and which are parallel to this card.
Also preferably, the means of storing electrical energy consists of a capacitor.
The invention also relates to the use of such electropyrotechnic igniters in gas generators for airbag modules connected in a motor vehicle by a bus-type hard-wired circuit to a central control unit.